I presume you mean 25 mm (25 millimeters) wide, not 25 mega-meters (25,000 km, which is about twice the diameter of the Earth), and that this is not a trick question about a shelf 1 m wide meaning deep, so the answer depends on what the length of the shelf is. A meter (I'll use my nation's spelling, you use yours!) is 1,000 mm, so we divide 1,000 by 25. The answer is 40.
One. After that it's not an empty shelf.
it depends on the shelf of course
2.5cm per 25mm
You can wind up with 10 different pairs of books in your hand, which you can choose from a shelf of 5 books in 20 different ways.
2.5*10-5 km in 25mm
The answer depends on how many books on each subject there are.
it depends on how big the books are.
That depends on how big the books are :P
25 mm = 0.025 meterVolume = 7,200 m2 x 0.025 m = 180 m3
120. You do 5*4*3*2*1=120. you multiply the number that you are given for example how many times can books 3 be arranged on a shelf you multiply 3*2*1=6 that is your answer
792 different groups of 5 books, in 95,040 different sequences.
There is no 25mm handgun. That is the size of an automatic cannon that is mounted on an armored vehicle.
5 books can be lined up on a shelf in (5 x 4 x 3 x 2 x 1) = 120 different sequences.